The present invention relates to a power roller bearing of a toroidal type continuously variable transmission and a method of manufacturing the power roller bearing of the toroidal type continuously variable transmission.
Speed change gears have mainly been used as conventional transmissions for vehicles such as automobiles. The speed change gears comprises a plurality of gears, and the engagement mode of the gears is varied to transmit torque from an input shaft to an output shaft. However, in the conventional speed change gears, torque is varied step-wise and discontinuously at the time of changing the speed. Thus, the speed change gears have drawbacks such as a loss in power transmission and vibration at the time of changing the speed.
Under the circumstances, a continuously variable transmission, in which torque is not varied stepwise or discontinuously at the time of changing the speed, has recently been put to practical use. In the continuously variable transmission, no vibration occurs at the time of changing the speed, and the loss in power transmission is less than that in the speed change gears. In addition, the continuously variable transmission is fuel-efficient when it is mounted in the vehicle. As an example of the continuously variable transmission, a belt type continuously variable transmission is mounted in some type of passenger cars.
On the other hand, as an another example of the continuously variable transmission, a toroidal type continuously variable transmission has been proposed. The toroidal type continuously variable transmission comprises an input shaft rotated by a drive source such as an engine, an input disk, an output disk, and a power roller bearing. The input disk is supported on the input shaft and rotated in interlock with the input shaft. The output disk is supported on the input shaft so as to be opposed to the input disk and is rotated in interlock with the output shaft.
A trunnion is provided between the input disk and the output disk so as to be rotatable about a rotational shaft. A displacement shaft is provided at a central portion of the trunnion. The power roller bearing is supported on the displacement shaft.
The power roller bearing comprises an outer race 62, as shown in FIG. 13B, which is supported on the displacement shaft of the trunnion, a power roller 61, as shown in FIG. 12B, which is rotatably supported on the displacement shaft, and balls serving as rolling elements rolling between the outer race and the power roller.
Both power roller 61 and outer race 62 have annular shapes. The power roller 61 and outer race 62 have raceway grooves 63 and 64 formed in their mutually opposed end faces. The raceway grooves 63 and 64 have annular shapes and arcuated cross sections. The balls roll along the raceway grooves.
The outer race 62 swings along with the trunnion. The power roller 61 has, as part of its peripheral surface, a traction face 65 put in rotational contact with the input disk and the output disk. In the power roller bearing, the outer race 62 swings along with the trunnion and the traction face 65 of power roller 61 is put in rotational contact with the input disk and output disk, so that a torque of the input shaft is transmitted to the output shaft with a desired acceleration ratio or deceleration ratio. The surface of raceway groove 63 and the traction face 63 of power roller 61 and the surface of raceway groove 64 of outer race 62 constitute raceway surfaces.
Compared to the aforementioned belt type continuously variable transmission, the toroidal type continuously variable transmission can transmit a higher torque. Thus, the toroidal type continuously variable transmission is regarded as efficient as continuously variable transmission for middle-sized and large-sized vehicles.
The toroidal type continuously variable transmission, however, is required to transmit a still higher torque. Accordingly, compared to general mechanical components such as gears and bearings, which receive repeated stress, the power roller 61 and outer race 62 of the power roller bearing suffer much greater repeated bending stress and repeated shearing stress.
The outer race 62 of the power roller bearing supports a thrust-directional load applied to the power roller 61 from the input disk and output disk. Consequently, the outer race 62 receives a very high stress at its raceway groove 64. Since the outer race 62 is supported on the displacement shaft, it receives a bending load due to the aforementioned thrust-directional load. A tensile stress due to the bending load acts on the outer race 62. In the state in which these stresses act on the outer race 62, the outer race 62 swings between the input disk and the output disk.
On the other hand, the power roller 61 transmits power from the input disk to the output disk in the state in which the traction face 65 is in rotational contact with the input disk and output disk and receives a great load from these disks. The power roller 61 thus swings repeatedly in the state in which a very high stress acts on the traction face 65 and raceway groove 63. In addition, a repeated tensile stress due to the above-mentioned high load acts on the raceway groove 63.
The power roller 61 and outer race 62 of the power roller bearing are required to have a long life, while a very high stress acts on the raceway surfaces such as the surfaces of raceway grooves 63 and 64 and the traction face 65.
In an example of a conventional method of manufacturing the power roller 61 and outer race 62 of the power roller bearing of the toroidal type continuously variable transmission, a rolled cylindrical solid material is cut and processed. Jpn. Pat. Appln. KOKAI Publication No. 9-126290 describes a method of manufacturing the power roller 61, wherein an annular material is cemented or carbonitrided and then forged.
If a metallic cylindrical material is cut and processed to obtain the power roller 61 and outer race 62, the yield of products decreases due to the cutting process and the time needed for processing increases. As a result, the manufacturing cost increases.
Besides, as shown in FIGS. 12A and 13A, a flow of metallic structure, so-called metal flow J, occurs along axis Ma and Mb at the time of rolling, etc. in cylindrical solid materials 60a and 60b which are formed as materials of the power roller 61 and outer race 62 through melting, forging and rolling steps.
If the power roller 61 and outer race 62 are formed by cutting and processing the materials 60a and 60b, the metal flow J occurs along axis Ma and Mb, as shown in FIGS. 12B and 13B.
Consequently, ends of metal flow, E1, E2 and E3, at which the metal flow ends, occur at the raceway surfaces, i.e. the surfaces of raceway grooves 63 and 64 and traction face 65.
If a stress, which is much higher than that on general mechanical components, acts on the raceway surfaces 63, 64 and 65, the power roller 61 and outer race 62 formed by the cutting process may easily be broken along the metal flow J. Accordingly, the life of the power roller 61 and outer race 62 formed by the cutting process is generally short. Thus the life of the toroidal type continuously variable transmission including these power roller 61 and outer race 62 is also short.
In the aforementioned method described in Jpn. Pat. Appln. KOKAI Publication No. 9-126290, the annular material with a metal flow extending along its axis is used. This material is cemented or carbonitrided and then forged along its axis. The material is then expanded toward its periphery into a shape corresponding to the power roller. At last, the material is cut and processed to obtain the power roller.
The power roller formed by this method, like the power roller 61 and outer race 62 formed by the above-described cutting process, has ends of metal flow at the raceway surfaces such as the raceway groove and traction face. Accordingly, the power roller manufactured by the method described in Jpn. Pat. Appln. KOKAI Publication No. 9-126290 has a short life in general. Thus the toroidal type continuously variably transmission including this power roller has a short life, too.